1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member. More particularly it relates to an electrophotographic photosensitive member having a photosensitive layer containing a resin of a specific structure. The present invention is also concerned with an electrophotographic apparatus, a device unit and a facsimile machine that have said photosensitive member.
2. Related Background Art
In recent years, a large number of electrophotographic photosensitive members using an organic photoconductive material have been proposed and practically used because of their advantages such as environmental safety, a high productivity, the readiness for material designing and the prospect of further improvements. For these electrophotographic photosensitive members, various properties such as electrical, mechanical and optical properties are required in accordance with the electrophotographic process. In particular, for the electrophotographic photosensitive members which are repeatedly used, the durability to the electrical or mechanical external force is required, which external forces directly act on their surfaces during corona charging, imagewise exposure, toner developing, transfer to paper and cleaning processes.
Specifically required for electrophotographic photosensitive members is having a durability to the deterioration of performances due to ozone generated during corona charging, i.e., a decrease in sensitivity, a decrease in potential, an increase in residual potential, as well as the resistance of the surfaces to the wear and scratches due to sliding friction during transfer and cleaning processes.
In general, the surface of an electrophotographic photosensitive member is made of a very thin resin layer, therefore the properties of the resin are very important. As a resin satisfying the above conditions, polycarbonate resins having a bisphenol-A skelton have been used. These resins, however, do not possess all of the necessary properties required for resins used in electrophotographic photosensitive members, and they have the problems as shown below.
(1) Poor solubility: They only show a good solubility in some of halogenated aliphatic hydrocarbons such as dichloromethane and 1,2-dichloroethane. Since the boiling points of these halogenated aliphatic hydrocarbons are low, photosensitive members manufactured using a coating solution prepared from any of these solvents tend to have whitened coatings. In addition, when such a solvent is used, it is not easy to control the solid contents in coating solutions.
(2) Other than the halogenated aliphatic hydrocarbons, the resins are partly soluble in tetrahydrofuran, dioxane, cyclohexane or a mixed solvent of these. These solvents, however, have a poor stability with time such that they may gel in few days, and are not suitable for the manufacture of photosensitive members.
(3) Moreover, even if the disadvantages stated in the paragraphs (1) and (2) are overcome, solvent cracking tends to occur in polycarbonate resins having only bisphenol-A or a bisphenol-A derivatives as a main chain skeleton.
(4) Furthermore, in the case of the conventional polycarbonate resins, a film formed from such a resin is poor in lubricity, which tends to cause fall faulty images due to the scratches made on the photosensitive member, faulty cleaning due to early deterioration of the cleaning blade and faulty cleaning due to turnover of a cleaning blade.
With regard to the solvent stability mentioned in the paragraphs (1) and (2), use of bisphenol-Z type polycarbonate resins (hereinafter "polycarbonate-Z resins") having a bulky cyclohexylene group has solved these problems. The polycarbonate-Z resins, however, show relatively large volume shrinkage especially when films are formed by casting using solutions, so that a stress often remains in the films. Hence they have had a disadvantage of a relatively weak resistance to stress corrosion. As a means for solving this problem, for example, Japanese Patent Application Laid-open No. 61-62040 discloses a method in which polycarbonate-A resin and polycarbonate-Z resin are mixed so that any cracking due to stress corrosion can be decreased. Japanese Patent Application Laid-open No. 61-62039 also discloses a method in which bisphenol-A and bisphenol-A are copolymerized so that any cracking due to stress corrosion can be decreased. Both the methods, however, are unsatisfactory means against the solvent cracking.
Besides this problem, as noted in the paragraph (4), conventional polycarbonate resins have a relatively low lubricity to a cleaning blade used in the electrophotographic process, which may cause turnover of the cleaning blade during running resulting in faulty cleaning, or scratches because of a strong force applied to the photosensitive member drum. Such disadvantages have been pointed out in the art. As a means for overcoming such disadvantages, a method is known in which silicone oil is added or in which, as disclosed in Japanese Laid-Open Patent Application No. 61-132954, polydimethylsiloxane blocks are copolymerized with a polycarbonate resin. However, the method of silicone addition has the disadvantages that it causes the deterioration of electrical characteristics in electrophotography, specifically, of sensitivity and residual potential, and that long-lasting lubricity cannot be obtained due to the loss of silicone oil on the surface layer with progressive running.
Use of the aforesaid copolymerization product of polydimethylsiloxane blocks can bring about a good lubricity. However, conventional polydimethylsiloxane copolymers have a problem such that a solution thereof becomes milky-white or gels and that their use in the surface layers of electrophotographic photosensitive members is not satisfactory in view of durability or running performance.
In addition, due to the demand for the high sensitivity of organic electrophotographic photosensitive members, low-molecular weight compounds such as a charge-transporting material are often added in a large quantity which may cause the separation of the low-molecular weight compound when electrophotographic photosensitive members are stored for a long period of time, bringing about the problem of layer separation.